Light filter



Dec. 3, 1963 J. P. HOXIE ETAL 3,113,033

LIGHT FILTER Filed May 16, 1.960 2 Sheets-She's: 1

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INVENTORS JOHN P HOX/E AND ALA/v J WERNER Dec. 3, 1963 Filed May 16.1960 J. P. HOXIE ETAL 3, ,033

LIGHT FILTER 2 Sheets-Sheet 2 73am A/JM/TTA/VC'E //v PsecawrWAVELENGTH-MILL/M/CRONS 0 INVENTORS .1 4. 5 JbH/v l? b ox/E A/vo ALA/vZ' WE NER Arra/Z/v United States Patent Ofiice 3,1 13,533 Patented Dec.31, 1963 3,113,033 LIGHT FTLTER John P. Hoxie and Man J. Werner,Corning, N.Y., as

signers to Corning Glass Works, Corning, N.Y., a corporation of New YorkFiled May 16, 1966, Ser. No. 29,331 2 Claims. (Cl. 1(i654) Thisinvention relates to the art of illumination and more specifically tothe provision of improved illumination for medical and surgicalpurposes. It is particularly concerned with a glass filter adapted totransmit light from an incandescent source in a color corrected formwith the non-visible heat radiations removed by absorption. The filtermay take any conventional form, such as a globe or closed cylinder,adapted to be mounted in a lighting Ware fixture to enclose anincandescent light source.

Incandescent lamps are particularly desirable inasmuch as they provide amore balanced or continuous spectral energy distribution and a closerapproximation to black body illumination than readily available are orfluorescent lamps. However, incandescent illumination presents twoserious problems, chromaticity characteristics outside specified limitsdue to low color temperature of the illuminant and infrared radiationsas a source of heat.

Conventional incandescent lamps employ as an illuminant a tungstenfilament operating at a color temperature of 2854" K., or slightlyhigher depending on lamp design. However, the chromaticitycharacteristics of a black body illuminant having a color temperature of3500 K. or higher are recognized as more satisfactory for many purposes,the human eye apparently being better adjusted to the making of colorjudgments with such illumination.

This is of particular significance in medical and surgical diagnosiswhere quick and/ or accurate color comparisons or discriminations mustfrequently be made in determining minor, but vital, changes in, ordifferences between, skin and/ or tissue colors. In recognition of thissituation, a Federal Specification, W-F-4l6, Surgical Lighting Fixtures,has been established to define acceptable illumination in terms ofchromaticity values. It requires that the conventional x value ofchromaticity be no greater than 0.40, the corresponding y value nogreater than 0.41.

A substantial portion of the radiation from a conventional incandescentlight source is in the infrared region of the spectrum. This may resultin severe overheating, particularly when concentrated by reflectors orother means employed to provide intense or spot lighting. It isdesirable therefore to provide a filter that will selectively remove theinfrared radiation and also color correct the transmitted visibleportion of the radiation in accordance with prescribed specifications.Such a filter should be composed of a heat resistant material having alow coefiicient of thermal expansion to avoid fracture from thermalstresses during rapid and/ or non-uniform changes in temperature. Thematerial should also be adapted to fabrication, as by press molding,into the shape of a hollow enclosure such as a cylinder or globe.

Heretofore, filters have been molded from low expansion borosilicateglasses containing ferrous oxide as an infrared absorbing agent. Suchglasses have undesirably high y chromaticity values, i.e. a greenishcolor character, when suflicient iron oxide is added for completeinfrared absorption. This greenish character cannot be adequatelyovercome with color additives to provide a satisfactory color correctingfilter. Even in the partial absorbing filters heretofore employed, the ychromaticity value has approximately the upper limit of 0.41 whereassomewhat lower values are considered more desirable as explainedsubsequently.

Attempts have also been made to use combinations of filter glassesfabricated in the form of flat sheets or plates.

The resulting multiple, flat plate filter assemblies are cumbersome,expensive to construct and difficult to maintain and use. It has notheretofore been considered possible to produce a heat resistant glasshaving substantially complete heat absorption, adequate colorcorrection, and adapted to molding as a hollow filter.

It is the primary purpose of this invention to provide an improvedfilter in which these various requirements are adequately met. A morespecific purpose is to provide a single, heat absorbing, colorcorrecting filter in the form of a hollow enclosure for an incandescentlight source. A further purpose is to provide a glass filter having alow thermal coefiicient of expansion that adapts it to use in a lightingunit operating at high temperatures. Other advantages and benefits ofthe invention will become apparent from the description that follows.

The invention resides in a light transmitting filter for incandescentillumination that absorbs infrared radiation from such illumination andcolor corrects the remaining transmitted portion to simulateillumination from a light source having a color temperature of at least3500" K. and chromaticity characteristics such that the x value is notgreater than 0.40, and the y value is not greater than 0.41, the filterbeing formed from a phosphate glass containing ferrous oxide as a heatabsorbing additive and an oxide of cobalt as a color correctingadditive.

It will be appreciated that the effectiveness of these oxide additives,both for heat absorption and color correction, is dependent on filterthickness as well :as on concentration of the oxides in the glass. It isdesirable to minimize the amount of glass additives both for economy andfor ease of glass melting and quality control. Accordingly, it iscustomary to employ filters having a thickness of about 6 millimeters.Subsequent reference to glass additive concentrations will be withreference to filters of such conventional thickness unless otherwiseindicated. It will be understood that corresponding effects can beachieved in filters of lesser or greater thickness by employingcorrespondingly greater or lesser concentrations of glass additives.

In general, there should be incorporated in a phosphate base glassfilter of conventional thickness about 15% iron oxide computed as FeOand about 0.005-0.015% cobalt oxide computed as C00 to provide anadequate degree of heat absorption and color correction. The pre ciseamount of each additive will depend on desired chromaticitycharacteristics as well as on filter thickness. However, the weightratio of FeO/CoO should exceed about to 1 in any event. The amount ofoxide additive to the base glass will depend on the oxidation level,i.e. equilibrium with higher oxidation states such as ferric oxide, inthe glass. Accordingly, some adjustment may be necessary betweenappreciably difierent base glasses and for variations in meltingconditions as explained later. However, such adjustments are readilydeterminable by one experienced in glass melting.

The invention is further described with reference to the accompanyingdrawing in which FIG. 1 is a view, partly in section, of a lightingfixture in accordance with the invention,

FIG. 2 is a graphical illustration of chromaticity characteristics, and

FIG. 3 is a graphical illustration of visible transmission through afilter of the invention.

FIG. 1 illustrates a conventional surgical lighting fixture embodying anincandescent tungsten filament lamp 1!), a metal capped, glass cylinder12 enclosing the lamp, and a generally elliptical reflector 14 adaptedto concentrate light in a focal plane. In accordance with the invention,cylinder 12 constitutes a heat absor bing, color correcting filter forradiations from incandescent source it). Depending on the particulartype of lighting fixture or application, the filter may take variousother physical forms such as a globe.

In accordance with the invention, light filter 12. is molded from aphosphate glass characterized by the presence of ferrous and cobaltoxides in amounts indicated above. In melting the phosphate glass fromwhich the filter is molded, mild reducing conditions are employed tomaintain the iron content of the glass in a reduced ferrous state. Suchreducing conditions may be maintained by conventional procedures, suchas the use of ferrous oxalate as a source of iron in the glass and/ oraddition of minor amounts of a reducing agent, such as starch, sugar, oran ammonium compound in the glass batch. In the absence of other glasscolorants, ferrous iron-containing phosphate glasses have a greenishcast or tint.

The term phosphate glass is here used in its conventional sense todenote a glass containing P as the primary glass-forming oxide andfurther containing glass modifying and flux materials, particularlydivalent metal oxides. Other conventional glass forming materials suchas SiO B 0 and A1 0 are present in compatible amounts for purposes ofimproved melting and working as well as stabilization of the glassagainst weathering. Easily reducible oxides, such as lead oxide, andcolorants other than those specified should generally be avoided becauseof their tendency to interfere with the color correcting effects ofcobalt and iron oxides. In general, glasses suitable for molding offilters in accordance with the invention, exclusive of the cobalt andiron oxide additives, consist essentially of 45-80% P 0 up to 20% of oneor more divalent metal oxides, preferably oxides of Mg, Zn, Ca and Ba;525% A1 0 up to 30% SiOg; up to 20% B 0 and, optionally, up to or moreof alkali metal oxides. The latter are generally avoided in the interestof maximum infrared absorption and lower thermal coeificient ofexpansion. A low expansion is desired to render the filter heatresistant and is preferably on the order of 50x 10* or below.

By way of further illustration, 2. number of specific glass compositionssuitable for present purposes are illustratively shown in the followingtable in terms of percent by weight together with their average thermalexpansion coeificients between 0 and 300 C. The latter is shoum as anumber which when multiplied by 10' gives the actual expansioncoelficient per degree C.

. but:

In melting phosphate glasses for present purposes, conventional batchmaterials and melting practices may be employed except as otherwiseindicated. The glass batches may consist of a mixture of oxides,phosphates, fluorides and a reducing agent properly proportioned toproduce the desired glass composition. The batch mixture may be meltedin a pot type melting unit at a maximum temperature of about 1400 C. fora time suificient to produce a completely fused and adequately finedglass for molding purposes. The molten glass is then cooled to asuitable working temperature and delivered to a glass press or othertype of molding equipment.

The color correcting characteristics of the present filter areillustratively described with particular reference to the graphicalillustration of FIG. 2. In this illustration, progressively increasingand y chromaticity values x are shown respectively along the horizontaland vertical axes of the graph. Horizontal line A and vertical line B onthe graph define the boundaries or limits established by SpecificationFW4l6 referred to earlier. Light from an illuminating fixture inaccordance with this specifica tion will have chromaticity valuesfalling below line A and to the left of line B. For adequatetransmission, it is generally desirable to keep both the x and y valuesabove about 37, i.e. the extreme left corner of the graph.

Curve C illustrates the relationship between the chromaticity values ofuncorrected light from Planckian or black body radiators at variousindicated temperatures. It will be observed that the values ofuncorrected light from a conventional tungsten filament having a colortemperature of 2854" K. is well to the right of the values acceptablefor present purposes.

We have discovered that the chromaticity of such incandescent light canbe corrected to bring it within accepted limits, as defined by lines Aand B, by including ferrous and cobalt oxides in a phosphate glassfilter in proper proportions. The color correction effects attainableare illustrated in FIG. 2. Chromaticity characteristics of incandescentlight transmitted through 6 mm. thick filter molded from glasses havingthe compositions set forth in the above table are shown by circled dotsidentified by numbers corresponding to the composition numbers in thetable. As indicated earlier, the chromaticity values are dependent onfilter thickness as well as on the ratio of colorant oxides in theglass. Thus, for glass filter thicknesses greater than 6 mm., therewould be a decrease in the x chromaticity value of a given glass.Correspondingly, thinner filters would be characterized by an increasedx value. The y chromaticity value is comparatively little affected bychange in thickness and the plotted chromaticity points may beconsidered as moving from right to left on the graph along an almosthorizontal line with increase in filter thickness. In general, anincrease in the cobalt oxide content of a filter will greatly decreasethe x chromaticity value and produce a relatively much smaller decreasein the y value. Increasing the ferrous oxide content tends to increasethe y value and decrease the x value. By proper control of ferrous andcobalt oxide contents, as well as filter thickness, chromaticity valuesmay be varied as desired in a given glass.

It has further been found that the optimum chromaticity characteristicsare not those of a true black body illuminant. Rather, characteristicshaving a slightly higher y value are more acceptable. These are definedby a polygonal area generally designated as D on the graph of FIG. 2.This area is roughly parallel to the black body illuminant curve butofiset along the y axis.

In addition to satisfying chromaticity characteristics or limits asexplained earlier, a color correcting filter should also provide acontrolled transmittance across the visible portion of the spectrum. Inthe lighting art, this is commonly defined with reference to visibletransmittance curves. Such curves are graphical plots of transmittanceat wavelengths across the visible portion of the spectrum, usually takenas 400750 millimicrons. 'For proper color correction to permit accuratecolor comparisons or discriminations, the transmittance curve should bea substantially smooth or unbroken curve although varying intransmittance at different wavelengths. Ideally, the transmittance curvewill progressively decrease from a high value at a low wavelength to alower value at a high wavelength with a substantially constant rate ofvariation. In other words, the curve should not be characterized bysharp or prominent dips, i.e. a plurality of maxima and minima values. Atransmittance curve for the glass shown as Example 3 in the compositiontable is plotted in FIG. 3 to better illustrate this characteristic ofpreferred filters in accordance with the invention. The curve showsspectrophotometric measurements of percent transmittance through a sixmm. thick filter across the range of visible radiations. Except forsmall dips of minor consequence, as at about 535 millimicrons, the curveis essentially smooth and unbroken, an indication of desired colorcorrection.

What is claimed is:

l. A phosphate glass, light transmitting filter for an incandescentlighting unit characterized by a combination of color correctionadditives consisting of about 1-5% ferrous oxide and about 0.005-0.0l5%cobalt oxide on the basis of a 6 mm. thickness of glass, the ratio offerrous oxide to cobalt oxide being at least 150:1 in the glass, theamount of ferrous oxide being sufiicient to absorb infrared heat raysfrom incandescent illumination and the cobalt oxide, in combination withthe ferrous oxide, being present in an amount effective to color correctthe transmitted portion of the illumination to simulate illuminationfrom a light source having a color temperature of at least 3500 K. andchromaticity characteristics such that the x value is not greater than0.40 and the 3/ value is not greater than 0.41, the phosphate glass, inaddition to the cobalt oxide and iron oxide additives, consistingessentially of 45-80% P 0 up to of at least one divalent metal oxide,525% A1 0 up to SiO and up to 20% B 0 2. A filter in accordance withclaim 1 having chromaticity characteristics within the area designatedas D in FIG. 2 of the drawing.

References Cited in the file of this patent UNITED STATES PATENTS2,194,784 Berger Mar. =26, 1940 2,278,501 Tillyer et al. Apr. 7, 19422,359,789 Pincus Oct. 10, 1944 2,441,853 Stanworth May 18, 1943 2,78,006 Kreidl et al May 29, 1956 2,900,264 Brown Aug. 18, 1959

1. A PHOSPHATE GLASS, LIGHT TRANSMITTING FILTER FOR AN INCANDESCENTLIGHTING UNIT CHARACTERIZED BY A COMBINATION OF COLOR CORRECTIONADDITIVES CONSISTING OF ABOUT 1-5% FERROUS OXIDE AND ABOUT 0.005-0.015%COBALT OXIDE ON THE BASIS OF A 6 MM. THICKNESS OF GLASS, THE RATIO OFFERROUS OXIDE TO COBALT OXIDE BEING AT LEAST 150:1 IN THE GLASS, THEAMOUNT OF FERROUS OXIDE BEING SUFFICIENT TO ABSORB INFRARED HEAT RAYSFROM INCANDESCANT ILUMINATION AND THE COBALT OXIDE, IN COMBINATION WITHTHE FERROUS OXIDE, BEING PRESENT IN AN AMOUNT EFFECTIVE TO COLOR CORRECTTHE TRANSMITTED PORTION OF THE ILLUMINATION TO SIMULATE ILLUMINATIONFROM A LIGHT SOURCE HAVING A COLOR TEMPERATURE OF AT LEAST 3500*K. ANDCHROMATICALLY CHARACTERISTICS SUCH THAT THE "X" VALUE IS NOT GREATERTHAN 0.40 AND THE "Y" VALUE IS HOT GREATER THAN 0.41, THE PHOSPHATEGLASS, IN ADDITION TO THE COBALT OXIDE AND IRON OXIDE ADDITIVES,CONSISTING ESSENTIALLY OF 45-80% P2O5, UP TO 20% OF AT LEAST ONEDIVALENT METAL OXIDE, 5-25% AL2O3, UP TO 30% SIO2, AND UP TO 20% B2O3.